The present invention relates generally to methods and system for monitoring of arterial parameters and more particularly to a method and system for continuous non-invasive monitoring of patient parameters.
Generally, non-invasive monitoring of arterial parameters is done using ultrasound measurement systems. However, continuous ultrasound measurements of cross-sectional area and volumetric flow rate of blood vessels using two-dimensional imaging are difficult to perform and typically only provide qualitative measures because of the inability to verify proper alignment of the ultrasound probe and determine the orientation of the ultrasound beam relative to the vessel. Current quantitative flow measurements use the long-axis (longitudinal) view of the vessel along with interactive measurement tools to manually correct for the Doppler angle (angle between the true blood velocity direction and ultrasound beam) using a preview image and calculate the true blood velocities. The calculation of a volumetric flow rate requires a manual measurement of the vessel cross-sectional area and assumes a certain velocity profile for the blood flow across the lumen. This technique requires several manual steps by a sonographer and is thus, impractical for continuous vascular monitoring. Thus, cross-sectional area measurements are currently performed by manually measuring the diameter in a long-axis view of the artery with a sonographer using visual feedback to maintain precise alignment of the probe with the center of the vessel. This approach requires a sonographer and is prone to alignment errors thereby, making the process impractical for continuous vascular monitoring applications.
Other methods to correct for the Doppler angle include cross-beam Doppler where multiple firing at the center of the vessel to triangulate the axial velocities into a two-dimensional (2-D) velocity vector. Again this method assumes a radial symmetric velocity profile with a specific shape, such as a parabolic profile.
Furthermore, other volumetric techniques include using blood velocity estimates in an entire volume to provide data for estimating the volumetric flow rate. One method includes integrating the velocity over a curved surface to remove the dependency of the volumetric flow rate measurement on Doppler angle. Another method includes using the 3-D blood velocity estimates to define the centerline in a volume. Neither of these methods that utilize volumetric acquisition are suitable for producing continuous waveforms over the cardiac period because of the long acquisition time for the volumetric color flow frames and the potential for significant motion over the frame time.
Accordingly, there exists a need for efficient non-invasive continuous monitoring of patient parameters in real-time.